This historical review written to commemorate the fiftieth anniversary of Becquerel's discovery of the first indications of radioactivity covers the scientific events surrounding radioactivity from 1896 to 1904. It stresses the experiments that advanced theories and knowledge of radioactivity. Beginning with Becquerel's initial work with uranium compounds, it traces Marie Curie's work with thorium and her discovery, with her husband Pierre, of polonium and radium. It then describes the work of J. J. Thomson, Ernest Rutherford, and Fredrick Soddy that elucidated the properties of alpha and beta radiation. It closes with the experiments of Rutherford and Soddy that led to the theory of radioactive transformations of the elements.